In most computer systems today, the smallest addressable unit of storage is an 8-bit byte. Data is typically transferred between the various components of a computer system in groups of bytes, such as words, doublewords and quadwords to meet bandwidth needs. If the data is to be used or evaluated correctly, it is necessary to know the correlation of the byte addresses to the value of the data, i.e., the most significant byte ("MSB").
Processor architectures that order data such that the lowest byte address corresponds to the highest order byte (leftmost) are called Big Endian. Architectures that assign the lowest byte address to the lowest order (rightmost) byte are referred to as Little Endian. Some processor architectures, such as the PowerPC processor manufactured by IBM corporation support both byte ordering schemes.
To illustrate the difference, consider the format of the word of data shown in FIG. 1 Data structure "n" consists of 4 bytes of data represented as hexidecimal characters 1A, 2B, 3C, 4D. When a processor, in Big Endian mode, stores the word in memory, the resulting storage map appears as shown in FIG. 2. If the processor was set up in Little Endian mode, the storage mapping would look as shown in FIG. 3.
When configured to operate in Little Endian mode, processors based on the PowerPC architecture present a slightly different storage map to the true Little Endian map as shown in FIG. 3. When presenting 1, 2 or 4-byte data transfers to the memory subsystem, PowerPC processors such as the 601, 604 and 620 chips will modify the lower 3 bits of the effective storage address creating the "pseudo-Little Endian" storage map as shown in FIG. 4. The address translation results in reversing the byte ordering within doublewords from the true Little Endian mapping. This creates a problem for input/output (I/O) devices that need access to architected storage areas. Therefore, there is a need in the art to permit I/O devices, which require a true Little Endian image of these storage facilities to be able to operate with PowerPC processors.